Power factor is an important factor when you size a diesel generator, especially for industrial and commercial loads that use electric motors, transformers, pumps, air-conditioning systems, and other inductive equipment.
In an AC electrical system, power factor shows the link between real power (kW) and apparent power (kVA). Real power is the part that does useful work. Apparent power is the total load the generator and distribution system must carry.
Power factor is written as:
Power Factor = kW ÷ kVA
A power factor of 1.0, also called unity power factor, means that real power and apparent power are equal. As power factor drops, more apparent power is needed to deliver the same kW.
If you are comparing kva vs kw, remember that kVA is the full load the generator must supply, while kW is the useful output. If you ask what is kva, it is the apparent load. If you ask what is kvar, it is reactive power, which does not do useful work but still affects the load.
Diesel generator sets are usually rated in kilovolt-amperes (kVA) and kilowatts (kW). Many industrial sets use a 0.8 power factor rating.
For example, a generator rated at 500 kVA at 0.8 power factor can supply:
500 kVA × 0.8 = 400 kW
That gap matters when you size the generator. A load with a low power factor draws more current to deliver the same useful power. That extra current increases the burden on the alternator and the electrical distribution system. It can also mean larger cables, switchgear, and other equipment.
Many loads are inductive. Electric motors, transformers, fluorescent lighting, pumps, compressors, and air-conditioning systems can all need reactive power as well as real power.
As reactive power rises, power factor falls.
Non-linear electronic loads can create another problem. They distort the current wave shape, so power factor is not just a phase shift between voltage and current.
When you size a diesel generator, you must look at the actual load, not just the total kW.
Generator sizing should include:
the order in which major loads start and run
A generator may look big enough if you check kW alone, but it may still lack enough kVA or alternator capacity for the real electrical load.
At EA Power Systems, we look at both real and apparent power when helping customers select diesel generators for commercial and industrial sites.
Understanding power factor helps the selected generator handle the load reliably and keep voltage, current, and system performance in range.
Question: Why can a generator that seems large enough in kW still be unsuitable for a site?
Short answer: Because the generator must handle both kW and kVA. If the loads have a low power factor, they may need more current and more kVA than their kW ratings suggest. That can put extra stress on the alternator, cabling, switchgear, and distribution gear.
Question: What does a 0.8 power factor rating mean for a diesel generator?
Short answer: It means the generator’s kW output is usually 80% of its kVA rating. For example, a 500 kVA set at 0.8 power factor can provide 400 kW of real power. Check both ratings when you choose a generator.
Question: Which loads are most likely to reduce power factor?
Short answer: Inductive loads such as electric motors, transformers, pumps, compressors, fluorescent lighting, and air-conditioning systems can lower power factor because they need reactive power as well as real power. Non-linear electronic loads can also affect power factor by distorting the current wave shape.
Question: Why is starting current important when sizing a diesel generator?
Short answer: Some equipment, especially motors, can draw much more current at start-up than during normal running. If this is not considered, the generator may experience voltage dips, swings, or insufficient capacity during load changes, even if the running kW total looks fine.
Question: What should be checked before choosing a diesel generator for an industrial or commercial application?
Short answer: Check real power, apparent power, power factor, starting current, motor starting method, load steps, load diversity, harmonics, and the order of major loads. These checks help the generator support the load reliably and keep the system stable.
